Battery packs may be configured from a plurality of battery cells. Adding battery cells in series can increase battery pack voltage while adding battery cells in parallel can increase the amp-hour rating of a battery pack. However, the amounts of charge stored in series battery cells can vary do to manufacturing variation. If the charge of battery cells is allowed to vary from battery cell to battery cell, it is possible that less than the total capacity of the battery pack is available to a battery pack load. For example, if one battery cell stores more charge than another battery cell, charging and discharging of the battery cell may be dictated by the lower capacity battery cell since it is likely to reach a battery cell voltage limit sooner than a higher capacity battery cell during battery cell charging or discharging.
Lithium-ion battery cells offer the capability of storing relatively large amounts of charge in a small package. However, lithium-ion battery cells have an open circuit voltage profile that can make it difficult to determine when it is desirable to balance charge between battery cells. Further, the open circuit voltage profile of lithium-ion battery cells can make it difficult to determine how much charge is drained or added to a battery cell when the battery cell open circuit voltage is used to determine when battery cells are out of charge balance.
The inventors herein have recognized the above issues and developed an approach to address them. Specifically, the inventors have developed a method for balancing charge between battery cells, comprising: sampling an amount of charge stored in first battery cell and a second battery cell at first time; sinking or sourcing an amount of charge from the first battery cell and the second battery cell after sampling the amount of charge stored in the first battery cell and the second battery cell; and balancing of charge of a first battery cell or a second battery cell after sinking or sourcing the amount of charge from the first battery cell and the second battery cell in an amount of charge related to a difference between the amount of charge stored in the first battery cell and the amount of charge stored in the second battery cell at the first time.
By determining whether or not a charge imbalance between battery cells is present at a first time when determining charge of a battery cell may be more reliable, and balancing charge at a second time when determining charge of a battery cell may be less reliable, charge between battery cells may be balanced more accurately so that more charge capacity of the battery may be used. For example, at lower battery voltages, it may be possible to more accurately determine a difference in charge between battery cells since battery voltage changes at a higher rate with respect to battery charge than for middle range battery voltages. However, it may be undesirable to balance charge between battery cells at lower voltages because some battery cells may be at lower voltage limits. But, after the battery cells have been charged, a difference in charge determined at a lower level of battery charge may be used at a higher level of battery charge so that charge may be removed or added to battery cells without pushing the battery cells into charging or discharging limits.
The present description may provide several advantages. Specifically, the approach may improve battery cell charge balancing by providing improved estimates of charge to be added or removed from battery cells. In addition, the approach provides for accurate charge balancing without the cost of high resolution voltage measurements.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.